A diimmonium compound as a near-infrared light absorbing material has been widely known (for instance, see Patent Documents 1 to 3), and has been widely used in a near-infrared light absorbing filter, a thermal insulation film, sunglasses and the like. However, among these compounds, the compounds having a hexafluoroantimonate ion, a hexafluoroarsenate ion or the like as a counter ion has been used for the reason of being comparatively superior in heat resistance, and above all, the compound having a hexafluoroantimonate ion as a counter ion has been used in many cases. However, a compound not containing these metals has been desired to be developed in the industrial field in which the use of heavy metals is regulated, particularly, in the field of electric material, because the compound containing antimony is categorized as a deleterious substance. As means for solving the problem, there is a method of using a perchlorate ion, a hexafluorophosphate ion and a fluoroborate ion as the counter ion, but these counter ions are insufficient, in consideration of heat resistance and resistance to moist heat. Furthermore, a compound using an organic counter ion such as naphthalenedisulfonic acid is proposed (for instance, see Patent Document 2), but a field in which the compound is practically usable is limited because molar absorptivity is low and the compound in itself takes on a green tinge. In addition, compounds using a trifluoromethanesulfonate ion, bis(trifluoromethane)sulfonic acid imide and the like have been known (for instance, see Patent Documents 1, 4 and 5), but it cannot be said that these compounds have sufficient heat resistance and resistance to moist heat, so that a more excellent compound has been demanded to be developed.
By the way, near-infrared light is used for a beam for remotely operating electrical machinery and apparatuses, so that machinery and apparatuses emitting near-infrared light may cause misoperation in electrical machinery and apparatuses installed in the periphery. It is therefore needed to install a filter having a function of shielding near-infrared light on the front surface of such machinery and apparatuses.
Among electrical machinery and apparatuses, a PDP (plasma display, panel) has been frequently used in large-sized televisions in recent years. The PDP generates visible light necessary for an image according to a principle of: applying voltage on a rare gas (such as neon and xenon) which is sealed in a cell sandwiched between two pieces of tabular glass to generate ultra-violet light; and making the generated ultra-violet light irradiate a luminous body coated on the surface of cell wall. However, the PDP also emits a harmful electromagnetic wave such as near-infrared light, an electromagnetic wave harmful to a human body and an orange beam (hereafter referred to as neon light) which is originated in neon gas and decreases red light, simultaneously with the emission of visible light. It is accordingly needed to equip the PDP with an optical filter which passes the useful visible light through itself but shields the harmful electromagnetic wave including the near-infrared light.
A near-infrared light absorbing filter to be used in an optical filter is produced by coating a compound which absorbs near-infrared light (a near-infrared light absorbing compound) on the surface of a transparent support or on the surface of a functional film such as a film for shielding an electromagnetic wave which is harmful to a human body (hereafter referred to as an electromagnetic-wave-shielding film) with the use of a polymer resin as a binder. There are many near-infrared light absorbing compounds in such a field of application, but a diimmonium compound or a combination thereof with another near-infrared light absorbing compound is often used because a diimmonium compound has a wide absorption wavelength region to the near-infrared light.
However, a conventional diimmonium compound has not only the disadvantage of the harmfulness as above described, but also has a problem on stability as has been pointed out that the diimmonium compound has insufficient stability on heat resistance and on resistance to moist heat in general, when a near-infrared light absorbing filter is prepared by coating the diimmonium compound on a resin film with the use of a resin as a binder.
Patent Document 6 discloses a technology for stabilizing a diiummonium compound by making a polymeric resin layer contain the diimmonium compound while controlling the amount of a solvent remaining in the resin layer to a specified ratio or smaller. The technology needs, however, some work of controlling the amount of the remaining solvent, so that a diimmonium compound has been desired to be developed which shows high heat resistance and resistance to moist heat when coated by a more general coating method and drying method. Patent Document 7 discloses an optical filter with near-infrared light absorptivity, which has a diimmonium compound with a bis(fluoroalkyl sulfonyl) imide anion or a tris(fluoroalkyl sulfonyl)carbanion contained in a coating layer of a transparent polymeric resin, and states that the diimmonium compound, particularly one having the bis(fluoroalkyl sulfonyl)imide anion, shows good heat resistance and resistance to moist heat with specific examples of the compound. However, the Patent Document does not describe the name of a specific compound, a specific production method, physical properties or an application example about the diimmonium compound having the tris(fluoroalkyl sulfonyl) carbanion. Further, there is a method of making a near-infrared light absorbing compound contained in a sticking layer for adhesively bonding a polymeric resin film to a support, as a method of making a near-infrared light absorbing compound held in a resin film. The method can make the near-infrared light absorbing compound contained in the sticking layer of a functional film such as an anti-reflection film and an electromagnetic-wave-shielding film which compose an optical filter, and accordingly has a great merit of cost reduction in comparison with the method of additionally forming a layer containing the near-infrared light absorbing compound, because one step can be eliminated from coating steps. However, the method has been considered to have a high technical hurdle because of aggravating heat resistance and resistance to moist heat more than the method of making the near-infrared light absorbing compound contained in a polymeric resin film or in a coating layer.    Patent Document 1: Japanese Patent Publication (KOKOKU) No. 07-51555 (Page 2)    Patent Document 2: Japanese Patent Laying Open (KOKAI) No. 10-316633 (Page 5)    Patent Document 3: Japanese Patent Publication (KOKOKU) No. 43-25335 (Pages 7 to 14)    Patent Document 4: International Publication Pamphlet WO2004/068199    Patent Document 5: International Publication Pamphlet WO2004/048480    Patent Document 6: Japanese Patent Laying Open (KOKAI) No. 2000-227515    Patent Document 7: Japanese Patent Laying Open (KOKAI) No. 2005-49848 (Pages 2 to 12)